32 IOWA ACADEMY OF SCIENCES. 



very prominently, while by the road clown the west side is a drain, cutting 

 deep into loess, without a trace of Carboniferous strata. A little east of this 

 ravine is another ravine crossing the road, cutting through loess and vari- 

 ous Carboniferous strata; here is an excellent illustration of unconformity, 

 for within a hundred feet the surface of the Carboniferous strata slopes 

 northward, in the direction of the present drainage, allowing the loess to 

 rest successively on clay shale, coal, fireclay and shale. 



Indianola is built on a hill thickly capped with drift, while a hill east of 

 Carlisle has shale, clay, and coal out-cropping in the road, even near the top 

 of the hill. 



To see the bearing of these illustrations of unconfornaity, let it be I'emem- 

 bered that the old surface was exposed to erosion during untold centuries 

 from the close of the Carboniferous Age till the "ice Age." In that long 

 period there was opportunity to cut out the immense valleys occupied for 

 ages then as now, by small streams. The unconformity of the drift on this 

 ancient surface reveals the direction of drainage in pre-glacial times. This 

 unconformity indicates that the more prominent ravines of the present lie 

 in pre-glacial ravines, though frequently on one side of the ravine. 



At pi-esent three rivers caiTy the surface water to the Des Moines. At 

 times in the spring these rivers are tilled till their flood plains are sub- 

 merged, but ordinarily they are nearly dry. Making what seems due 

 allowance for high water iu spi'ing, one cannot help but wonder how these 

 streams could cut into Carboniferous strata or even wash away drift 

 material till each little river had such broad flats as those to be seen north 

 of Greeubush on North river, at Summerset on Middle river, and south of 

 Indianola on South river. 



Comparing the ravines that open into these rivers, we notice that where 

 the surface rocks are least easily decomposed there the sides of the ravines 

 are steepest and out-crops most easily found, while in sections where the 

 surface rocks are soft, as north of Lathrop, there the sides of the I'avines 

 are rounded and out-crops less frequently found; yet over it all the loess is 

 generally undisturbed. Some of these main ravines cut deep into loess, 

 while the same deposits are apparently as deep on the knolls that separate 

 parts of the ravine. Back from the main ravines reach the smaller ones, 

 rarely cutting deep enough to remove anything but loess. 



East of Buffalo bridge a valley nearly a quarter of a mile wide is cut 

 through a hill fully a hundred and seventy feet high composed of masses of 

 limestone, but the ravine mentioned now contains a stream nearly dry the 

 larger part of the year. What little water there is in this gorge flows north- 

 ward. 



Comparing the valleys running to the north with those running south- 

 ward there is nothing to indicate that one set has been favored more in its 

 formation by either ice or water from melting ice masses. We should natur- 

 ally expect ice moving from the northeast to gouge out the soft material 

 lying on the north slopes near the tops of the hills; yet such material is still 

 found exposed. At the unconformity mentioned where the ravine opens to 

 the northward the strata referred to are very exposed to such erosion. The 

 valleys sloping to the northward have no characteristics in common, distin- 

 guishing them from valleys sloping southward. Especially is it difficult to 

 conceive how ice, or water from a melting ice mass, could erode such a 



